366 



DISTURBANCES CAUSED BY TRAP ERUPTIONS. 



Silurian System, are developed to some extent. Now these contiguous rocks lie precisely between 

 the points of eruption of Blaen dyffrin and Castel cogan, and range in lines parallel to the fissures 

 of eruption. 



We thus obtain additional proof that the violent dislocations of the Silurian System, 

 and the lower strata of the Old Red Sandstone of Caermarthenshire, are connected with 

 these linear eruptions of trap, which here, as in Salop, Montgomery, Radnor and 

 Brecon, have determined the chief direction of the sedimentary masses through which 

 they penetrate. 



The strata along the line of eruption near Castel cogan, are extensively fractured, and the asso- 

 ciated beds of the Silurian System (hard quartzose grits), are in some places not only vertical but 

 actually reversed, as in cases formerly pointed out, dipping 70° to the north-west, and covered by 

 the Old Red Sandstone, dipping 30° to 40° south. (See PL 34. f. 11.) 



Mines. 



The chief mining district in Caermarthenshire called Nant-y-moen, the property of Earl Cawdor, 

 is about seven miles north of Llandovery. It not being my intention to give the details of 

 this valuable mine, I will merely consider its chief relations to the rocks in which it is situated. 

 The strata between Llandovery and Nant-y-Moen belong to the Cambrian System, and consist 

 of dark, thinly foliated shale, enclosing, though very rarely, concretions of quartzose conglo- 

 merate, the true "greywacke" of the German miner. Rising abruptly in the midst of this 

 shale is a mural ridge of unbedded, and apparently in parts brecciated, grey quartz rock, penetrated 

 by many veins of white pure quartz. This ridge, called Cerrig mwyn, strikes from north-east to 

 south-west, and is therefore parallel to the leading direction of all the stratified deposits of this 

 region. On the immediate flanks of Cerrig mwyn, the shale is much indurated, and associated 

 with black and white grits, the whole of which are highly veined, whilst in receding on each flank 

 from the ridge, the black shale resumes its regular appearance. 



All the mining ground now in use is situated in the north-western flank of this wall of quartz 

 rock. It is, therefore, very analogous to the Stiper Stones, p. 268. 



When we consider how many ridges of eruptive trap have been pointed out, ranging precisely in 

 this direction, and that they have often been accompanied by such quartz rock, it is no strained hy- 

 pothesis to suppose, that Cerrig mwyn is also an altered rock, the trap which produced the change 

 being at no great depth beneath the surface. For without quitting Caermarthenshire, the descrip- 

 tion already given of the quartz rock of Cairn-goch, as resulting from the eruption of the trap 

 of Blaen dyffrin-garn, goes far to explain the phsenomena, for there the trap is only just discernible 

 in a small obscure boss, while the mass of altered rock forms a conspicuous overlying ridge. 



There are three principal veins called the master, the red, and the comet. The two first range 

 more or less from north-east to south-west, parallel to the wall of quartz grit, and in this respect 

 they differ from most of the veins around the Salopian trap rocks. The third crosses obliquely, and 

 intersects the red vein. The master vein has been worked only at a high level, near the edge of the 

 protruding wall of quartz rock which flanks it. The other veins are wrought by lower levels driven 

 into the hill which slopes to a small brook, and none of these have yet reached within fifteen fathoms, 

 horizontally, of the edge of the parallel of Cerrig-mwyn. In the master vein, numerous strings of 



